Gotta say I'm running a Murkablo, Golden Cloudserpent Hatchling and planning to level my baneling. (Though I've got a few pets I can swap the Baneling out for of the water or flying type if I need to.)

The best thing I can suggest is to never underestimate damage mitigation. Pets that can shield/dodge/heal are typically way, way better than the others. Abilities that attack and mitigate are really good; cocoon strike that a lot of moths/crickets have, for instance, or the fly up and be immune abilities some flyers have. If you can be immune to an attack for one round while dealing damage, you've picked up some pressure against the opponent.

The really brokenly powerful ability, I've found, is Reflection. I've got my rare Flayer Hatchling to 25, and I'm working my shimmering wyrmling up there too. Reflection is really strong on base principles; it always fires first (yes, even ahead of "always goes first" moves), will prevent any effects targeting your pet whether damage or debuffs, and will cast the same thing on the enemy. Enemies typically aren't resistant to their OWN type, so you generally get decent damage. You can also "reflect" a heal on yourself, or a ground-based effect.

The really crazy bit is if you reflect one of the other damage-avoidance moves. Reflecting a Cocoon Strike means you've gotten two free turns. Reflecting the two-turn fly-up-and-then-attack move is broken. You fly up. You don't fly down. You can still attack freely. They cannot hit you. This lasts the whole fight. I've gotten lucky on some trainer battles and had my Flayer Hatchling take out three level 25 rares and finish the fight at full health, this way.

I've also gotten really good results out of my Shale Hatchling. Get the leeching bite, the web, and the damage reduction. Pop the damage reduction, web them, bite them. On most enemies, the double-heal-on-webbed-targets, when combined with the damage reduction, is enough to keep you topped off. It's not a fast strategy, but the sucker outlasts almost anything. Even if the bite or web are less effective, it doesn't affect the amount healed; the only issue I've had is with enemies using attacks that are strong against Elementals.

I've got a dozen or so 25s now, but I haven't settled on a "team". I swap out as needed by the fights. Those guys are my standard ringers, though, when I need to guarantee a win against high-powered pets while leveling a new guy.